Floyd Mayweather Jr will come out of retirement to box UFC star Conor McGregor in August, but Saul 'Canelo' Alvarez and Gennady Golovkin are unenthused.

After constant posturing and trash-talking, Mayweather and McGregor finally agreed to a bout in Las Vegas on August 26.

The fight has been met with excitement among fans but criticism from boxers, including former professional Oscar De La Hoya, who branded it an "embarrassment" before plans were finalised.

Mayweather-McGregor will take place three weeks before the unbeaten Golovkin defends his WBC, WBA and IBF middleweight titles against Canelo in the same city on September 16.

And the Kazakh believes there is only one boxing match fans should get excited about.

"Every day there's a show in Vegas and everybody understands the true boxing fight is September 16, not before," Golovkin said following a media conference in London.

Canelo asserted that his meeting with Golovkin is the only "real fight" on the calendar at the moment.

"I really don't want to comment about that fight. Our fight is a real fight, it's a fight you demanded and wanted," said Canelo, via a translator.

"It's two fighters at their best, at their peak, and you know what you're going to get."

Mayweather inflicted the only defeat of Canelo's 51-fight career, but the Mexican feels he has developed hugely since their encounter in September 2013.

Asked who has been the toughest opponent of his career, Canelo replied: "Mayweather. Mayweather's style doesn't make for attractive fights, it's difficult to land punches. He's not aggressive, he doesn't punch hard at all. I learned a lot [from that fight] and I've grown."